compare current field with the following to perform a subquery in MYSQL

I have a query, in it, I have a subquery, and I only want to execute the subquery always when the current code field does not repeat with the next.

I will show a sql pseudocode so that I understand what I want to do, especially in the CASE:

SELECT 
   @rownum:=@rownum+1 AS nro,
    0 AS 'Encab', 
   pa.codigo, 
   pa.partida,      
CASE 
WHEN pa.codigo = pa.codigo siguiente THEN '' 'aqui deseo comparar el pa.codigo actual con el siguiente'
ELSE 
(SELECT SUM(presupuestosa.valor + presupuestosa.valorm) FROM presupuestosa  
WHERE presupuestosa.obra=18
AND presupuestosa.empresa=1 AND presupuestosa.codigo=pa.codigo)   
END AS tvalor
FROM presupuestosa pa , (SELECT @rownum:=0) r WHERE pa.obra=18
AND pa.empresa=1 ORDER BY codigo ASC,Encab DESC,partida ASC

I have tried to make this solution by placing indicators to the repeated codes, but it has not worked for me. Thank you in advance.